1. Field of the Invention
The present invention relates to an optical receiving element, and particularly to a photodiode used in an optical communication system.
2. Description of the Related Art
In the optical communication, an electrical signal is converted into an optical signal at the transmitting end using a light emitting element and then transmitted through a transmission line, such as an optical fiber. The converted optical signal is converted back into an electrical signal at the receiving end using a light receiving element, such as a photodiode. Most widely used photodiodes have a mesa type structure.
FIG. 1 is a cross-sectional view of a conventional photodiode 10 having a mesa type structure. As shown in FIG. 1, the non-doped InGaAs and p-InP are sequentially stacked at one end of an InP substrate 11 by means of a single crystal growing process. An u-InGaAs absorption layer 12 of a mesa type and a p-InP window layer 13 are formed by etching. Thereafter, silicon nitride(SiNx) is stacked on the p-InP window layer 13 so that an insulation layer 14 is formed, and a predetermined portion of the insulation layer 14 is etched so that a part of the p-InP window layer 13 has an opening. A p-type electrode 15 is provided on the open portion of the p-InP window layer 13. Meanwhile, a non-reflection coating is applied to the position corresponding to the p-InP window layer 13 on the other side of the InP substrate 11, so that a light receiving region 17 having a predetermined size and an n-type electrode 16 are formed.
In the method of fabricating a photodiode of a mesa type as described above, the non-doped InGaAs and p-InP layers are stacked through the single crystal growth process. Further, the undesirable spreading of a p-type dopant, such as Zn or Cd, is not necessary.
The conventional photodiode having the mesa type structure, however, has several drawbacks in that the non-doped InGaAs and the p-InP are formed as layers in the form of a mesa type, then exposed to the atmosphere. As such, the non-doped InGaAs and the p-InP materials can be oxidized during a fabrication process. This oxidation may cause a deterioration in the quality of the optical element. In addition, a current leakage may occur on the surface that is mesa-etched, i.e., the surface facing the insulation layer, thereby reducing the life of the optical component. Moreover, the InGaAs film whose energy band gap is small, may have a larger current leakage, thus further deteriorating the reliability of the optical circuit.
Furthermore, in an ultra high-speed optical communication, the capacitance of the optical circuit must be small which can be achieved by reducing the spreading region of the p-InP. The photodiode of a related art, however, makes wire bonding by providing a p-type electrode connected directly to the spreading region of the p-InP. Therefore, it is difficult to reduce the area of the spreading region of the p-InP.